The present invention relates to a method for starting up a converter drive for an electric-motor, in particular an asynchronous machine, the inverter part of which is equipped with commutating capacitors for phase-sequence quenching and which is fed via an intermediate link with a d-c current impressed via an intermediate link by an intermediate link inductance from a controlled rectifier where a short time before or immediately with the beginning of the inverter timing, a charging voltage is applied to one, several or all commutating capacitors, and subsequently, the intermediate link current is increased from a low starting value to the desired reference value.
The known, self-commutating converter to which the invention relates is shown in FIG. 1. The course of the commutations in the inverter part WR is described, for instance, in ETZ-A, Vol. 96, (1975) No. 11, on pages 520 to 523. For a further explanation of the operation, FIG. 2 shows part of the circuit in the form of an equivalent circuit diagram for the instant of the capacitor charge.
The commutations in the inverter part WR of the converter are made possible by the magnitude and the sign of the charges of the six commutating capacitors C13, C35, C15, C46, C62 and C42 shown in FIG. 1. which are arranged between two phases each in the upper and lower half of the inverter. It can be shown that it is not necessary to charge all commutating capacitors before starting up such a self-commutating inverter to the condition which corresponds to the normal operating point. It is rather sufficient, for instance, to pre-charge a capacitor and/or to make sure that all capacitors can charge up by themselves after the inverter is switched on. As a rule, however, the value of the intermediate link current must be kept so small at the beginning of the starting-up that the former can be commutated already with the aid of the first capacitor charge which may still be rather small. In the course of the subsequent commutations, this starting charge is distributed not only to the remaining capacitors. Rather, the charging state of all capacitors is improved by reversal processes internal to the inverter, especially by the gradual increase of the intermediate link current and by means of reversals internal to the inverter in such a manner that the normal operating state is finally reached. The maximally permissible value of the intermediate link current at the beginning of the start-up can, of course, be chosen to be larger, the more closely the pre-charge corresponds to the charging state necessary for the normal operating point.
For pre-charging, some circuits and operating methods which require no additional measures circuitwise have already become known.
Thus, DE-OS No. 29 52 324 describes a device, by which only one commutating capacitor is pre-charged in one-half of the bridge with the correct polarity. To this end, the charging voltage is taken off via a two-pole switch and a limiting resistor or via two semiconductor switches with a limiting resistor each, directly at the output of the controlled rectifier.
Of particular interest, however, are operating methods which avoid additional component costs and make do with the control and regulating devices which are already available in intermediate-link converters.
Thus, a method for "triggering" a converter with phase sequence quenching is known from German Patent No. 29 52 323. The inverter feeds an asynchronous machine at the output of an intermediate-current link converter. With the start-up, the maximally permissible intermediate- link current is limited upon starting-up initially to a small value and the inverter is at the same time timed with an increased frequency. The value of this frequency is obtained from the oscillation frequency of a tuned circuit formed by the commutating capacitors and the motor strand inductances. After a sufficiently high voltage has built up at the capacitor, the frequency is increased or decreased to the values provided for normal operation. The voltage naturally present in the intermediate link at the output of the d-c control element or controlled rectifier on the input side is utilized at the instant of starting-up for a first pre-charge of the capacitors.
In a further method that has become known from the Japanese publication with the File No. 51-109 950 in connection with "Patent Abstracts of Japan" Vol. 2, No. 70, page 2575E 78 the inverter is acted upon before it is put in operation, briefly at its input by a high voltage. For this purpose a reference value for the control device of the rectifier which can be controlled on the input side is temporarily increased in step-fashion. The pre-charge of the commutating capacitors is made possible by the simultaneous firing of one rectifier of the inverter half connected to the positive or negative potential, respectively. After the pre-charging voltage is removed and the intermediate-link current has decayed, the inverter operation is started with a small starting value to an intermediate link reference value which increases up to the desired end value.
However, it has been found in practice, that pre-charging the commutating capacitors or setting the converter in operation with limited intermediate-link current is not sufficient in all cases to reach the stationary operating point of the converter. Rather, the charging voltage that can be applied to the commutating capacitor may be so small that the charge caused thereby cannot commutate the starting value of the intermediate-link current occurring at the instant of starting up. In addition, the effects of the starting and pre-starting of the converter on the connected work-performing machine is neglected in all known devices and methods.